By tagging them, the researchers hope to show just how far these birds fly.

"We know that the godwits leave New Zealand, and we know that they arrive in the Yellow Sea, but we really haven't had any information in between that," Dr Battley explains.

"We don't know whether they make stop-offs on the way; and if they do, where they are. And we don't know anything about the security of those sites - how safe they are from development."

Back in March and April, after their summer sojourn, 13 satellite-tagged bar-tailed godwits left New Zealand to fly north.

Six of those birds flew directly to South Korea, China, and Japan, flying more than 10,000 kilometres (6,000 miles) in just over a week.

With some of the other birds, the tracking device fell off en route, and in one case a bird island-hopped his way up to the Yellow Sea. "We suspect this is because he doesn't like having a 'matchbox' strapped to his back," says Dr Battley.

The direct flights are some of the longest migratory bird flights ever recorded - and some of the toughest.

Seabirds feed and rest on their long journeys, swifts feed whilst in flight; but for the godwits, says Dr Battley, it is essentially a non-eating, non-drinking flight. They also fly pretty well true.

"Their navigational expertise must be very high because the distances they were actually flying, as best we can estimate from the data, are not greatly different from the shortest possible distance between those two spots," the researcher says.

Also the godwits are very faithful to the stopover sites they use. One godwit, for instance, looks likely to have only two stopovers on the whole round trip: Yalu Jiang in China and then the Yukon Delta.

"It really reinforces how critical these sites are to these birds," says Dr Battley.

"If something were to happen to that area, [the godwit] may not know any of the other areas around that, and that would put it at a disadvantage in trying to find food."

Under threat

And the godwit population is in decline.

New Zealand hosts 70,000 godwits each summer, but it used to be home to 100,000. It is the same throughout the East Asian and Australasian flyways, where up to 85% of the shorebird populations are declining.

Just what is going on is part of what the godwit tracking and the broader Pacific Shorebird Migration Program, a joint initiative between the US Geological Survey and PRBO Conservation Science, hopes to find out.

The increasing reclamation of tidal mud flats in Korea and China, and the changes in geography due to projects such as the Three Gorges Dam, are part of the problem, Dr Battley suggests.

"Everywhere is under threat to a certain degree. You just have to zoom in on any spot around the Yellow Sea [on Google Earth] and it's getting reclaimed in one way or another."

But one thing the godwits are probably not is a potential carrier of avian influenza, scientists believe.

"It's clear these birds are going straight up to their destination and that's where they are stopping and then moving on," says Dr Battley.

"It's not like they are going around the coast of Southern China and stopping off in lots of places where they have the potential to pick this up."

Four of the tagged godwits have now left Asia for the 5,000km (3,000 mile) journey to their breeding grounds in Alaska.

That's where they will stay for the Northern Hemisphere summer before returning in September to New Zealand.